This application claims the priority of Germany patent document 198 11 187.8, filed Mar. 14, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust gas turbocharger with a variable turbine geometry, and more particularly, to a turbocharger which, in the engine firing operation, can be adjusted by way of an automatic turbine controller to a definable desired supercharging pressure in the intake port and, in the braking operation, can be adjusted by way of an engine braking system with an automatic braking controller as a function of operating parameters of the internal-combustion engine into a ram position which increases the pressure in the exhaust gas system, a change-over element being provided for the change-over between the engine braking system and the automatic turbine controller.
DE 195 43 190 A1 describes an engine braking system for a supercharged internal-combustion engine whose exhaust gas turbocharger has a variable turbine geometry with adjustable guide baffles. The guide baffles have guide blades which, by way of an adjusting member, can be adjusted such that the flow cross-section of the flow duct in the exhaust gas turbocharger is changed. As a result, depending on the operating condition of the internal-combustion engine, pressures of different degrees can be implemented in the section between the cylinders and the exhaust gas turbocharger. Thereby, the power of the turbine and, therefore, the compression performance of the compressor can be adjusted as required. The adjusting member acting upon the guide blades is controlled by an engine control and characteristic diagrams stored therein.
In order to achieve a braking effect in the braking operation of the internal-combustion engine, the guide baffles are changed into a ram position, in which the flow cross-section of the flow duct is clearly reduced. In the section between the cylinders and the exhaust gas turbocharger, a high excess pressure is built up. Simultaneously, the exhaust gas flows at a high speed through the ducts between the guide blades and acts upon the turbine wheel, whereupon the compressor builds up an excess pressure in the intake system.
The cylinder is therefore acted upon on the input side by an increased supercharging pressure. In addition, despite the opened-up decompression valves, the pressure waves generated by the opening of the exhaust valve cause a residual charge effect. That is, the pressure waves, for a short time, change the exhaust valves of an adjacent cylinder, which is just running at the end of an intake stroke, into an opening position and increase the initial compression pressure in the adjacent cylinder by the flowing-in exhaust gas to a clearly higher level than would be possible by means of the compressor alone. In the braking operation, the piston must therefore carry out more compression work in the compression stroke, whereby a strong braking effect is achieved.
DE 195 43 190 A1 defines a desired engine braking function in the braking operation. The guide blades or blocking bodies, which can be moved into the spaces between the guide blades, are adjusted until the ram pressure required for the desired braking effect is reached in the exhaust gas system between the cylinders and the exhaust gas turbocharger.
WO 96/39573 A1 describes an exhaust gas turbocharger with a variable turbine geometry, in which the adjusting member acting upon the adjustable guide blades of the charger is influenced by an automatic control unit. The automatic control unit comprises an automatic turbine controller and an engine braking system with an automatic braking controller. A change-over element causes a change-over to take place between the automatic turbine controller and the engine braking system. This automatic control unit contains, however, a large number of different automatic control components, and has a complicated and inflexible construction.
An object of the present invention is to provide a variable exhaust gas turbocharger. In particular, the turbocharger of the present invention is to be equipped at low expenditures with a braking function which can be flexibly adapted to the respective requirement.
According to the present invention, this problem has been solved by providing that the automatic turbine controller and the engine braking system each have a modular design and a separate construction, and as an alternative to the automatic braking controller, a manual braking signal can be supplied to the engine braking system, which signal can be generated in a manually adjustable brake operating device. A change-over element is provided for the change-over between the automatic braking controller and the brake operating device, and the signal generated by the manual brake operating device can be compared in a comparison element with a rotational-speed-dependent braking limit value.
The division into two separate modular systems with one automatic controller respectively, one for the engine firing operation and one for the braking operation, has the substantial advantage that exhaust gas turbochargers which are already used in an internal-combustion engine and are not yet equipped for the braking operation can be used at minimal expenditures also for the braking operation. An additional engine braking system with an automatic braking controller must only be provided in which the characteristic diagrams and parameters relevant to the braking operation are stored. An already installed automatic turbine controller, by way of which the engine firing operation is automatically controlled, must be changed just as little as the other structural elements required for the automatic control, such as sensors or adjusting elements which normally exist anyhow for the automatic control of the firing operation and can now also be used for the automatic braking control.
It is another advantage of the present invention that with a simple exchange of the automatic braking controller or of the engine braking system, various automatic control algorithms and characteristic diagrams or parameters influencing the automatic control can be implemented irrespective of the engine control employed. For example, automatic braking controllers which are adapted to various turbine geometries, whereby the variability is increased, can be provided and used.
Furthermore, a manually generated braking signal can be supplied to the engine braking system by way of a change-over element. The braking signal can be generated in a manually adjustable brake operating device, such as a steering column hand brake lever. As required, as an alternative to the automatic braking controller, a manually adjustable braking torque can be generated. A comparison element with a rotational-speed-dependent braking limit value is preferably also connected to the engine braking system in order to ensure that with a manual demand, the permitted braking power maximum is not exceeded at the current rotational engine speed so that the turbine geometry will not be critically stressed.
In order to be able to change between the braking operation and the engine firing operation, a change-over element is provided. Thereby, on one hand, the automatic turbine controller can be assigned to the engine operation and, on the other hand, the automatic braking controller can be assigned to the braking operation. The engine braking system can thus be coupled to an already existing automatic turbine controller. This feature results in the additional advantage that with only one coupling, both braking possibilities, i.e., the braking by way of the automatic braking controller as well as the manual demand of the braking performance, can be connected.
The braking function which can be set in the automatic braking controller is advantageously a constant braking torque so that the vehicle is decelerated in a constant manner, or a constant speed, that is, a cruise control. In the latter case, for determining the current speed, the transmission output rotational speed and the engaged gear are expediently determined. These quantities can be measured or computed by the existing automatic vehicle engine control system, and the automatic vehicle engine control system can also carry out the desired-actual comparison of the speed.
A comparison element for comparing the control signal generated by the automatic braking controller with a definable maximal braking signal is preferably connected behind the automatic braking controller. The comparison element takes over the function of an electronic stop in order to ensure that no mechanical overloading takes place as the result of an excessive pressure buildup.
Furthermore, a delay element can be arranged behind the automatic braking controller. The delay element has the function of transmitting control signals of the automatic braking controller with a delay in order to prevent pressure peaks in the cylinder which may occur if a high excess pressure is built up while a throttle valve arranged on the cylinder is still closed. During the switching-on of the turbine geometry adjustment and the switching-off of the throttle valve, the delay element provides a delayed transmission of the control signal. Thereby, the throttle valve will not still be closed when pressure is already being built up in the exhaust gas system.